This invention relates to an air-fuel ratio control method applied to an internal combustion engine having spark plugs with heaters, and more particularly to a method of this kind applied to an internal combustion engine to which is supplied a fuel containing methanol, which is poor in atomizing characteristic at a low temperature.
Conventionally, a control system has been proposed e.g. by Japanese Patent Publication (Kokoku) No. 64-8182, which is provided for an internal combustion engine using a mixed fuel of a plurality of kinds of fuels, for correcting the amount of fuel supply, the ignition timing, the amount of recirculation of exhaust gases, etc. depending on the composition of the mixed fuel.
However, when an internal combustion engine using a mixed fuel containing methanol is started at a low temperature, the mixed fuel connot be readily ignited by the spark plug since the mixed fuel is poor in atomizing characteristic at a low temperature. That is, the mixed fuel causes degraded startability of the engine.
Further, if, in order to improve the startability of the engine using a mixed fuel containing methanol when the temperature is low, the amount of fuel to be supplied at the start of the engine is increased irrespective of whether or not the fuel is sufficiently atomized, as is usually carried out in an internal combustion engine using gasoline as fuel, which is excellent in atomizing characteristic, the resulting air-fuel mixture becomes overrich which causes adherence of fuel to the spark plug and hence difficulty to ignite the air-fuel mixture.